Electron emission includes field electron emission, secondary electron emission, and photoelectric emission, as well as thermionic emission. A cold cathode is the cathode that performs electron emission by field electron emission, which occurs due to a tunnel effect when a strong electric field (109 V/m) is applied to the vicinity of the surface of a substance to lower the potential barrier on the surface.
The cold cathode does not require heating as does the hot cathode. Its current-voltage characteristics can be approximated by the Fowler-Nordheim equation. The electron emission portion, to which a strong electron field is applied while maintaining insulation, is structured (e.g., a needle structure) so as to increase the electric field concentration constant.
Early cold cathodes are of a diode tube structure employing a needle-like single crystal such as a whisker that is polished by electrolytic polishing. Recently, microfabrication techniques in integrated circuit or thin film technologies have resulted in significant advances in the manufacture of field emission electron sources (field emitter arrays) that emit electrons in a high electric field. Thus, electric field emission cold cathodes with extremely minute structures are now being manufactured.
This type of field emission cold cathode is the most fundamental electron emission device of all the major components of a ultra-small triode electron tube or an ultra-small electron gun. Structural miniaturization has resulted in such an advantage that the device can provide a higher current density than the hot cathode as an electron source.
Field emission displays (FEDs) using the cold cathode are expected to find applications in self-emissive flat panel displays, and research and development of electric field emission electron sources are actively underway.
The operation of and methods of manufacturing the electric field emission electron source have been disclosed in a research report by C. A. Spindt et al. of the Stanford Research Institute, which was published in the Journal of Applied Physics, Vol. 47, No. 12, pp. 5248–5263 (1976); in U.S. Pat. No. 3,665,241 issued to C. A. Spindt et al.; and in U.S. Pat. No. 4,307,507 issued to H. F. Gray et al.
The electric field emission electron sources known from the above publications are all equipped with a protruding electron emission portion, which is formed on a semiconductor or metal substrate. Around the emitter is formed a gate for applying an electric field to draw electrons. Electrons which are emitted from the emitter by applying voltage to the gate travel to an anode formed above the emitter, as shown in FIG. 8(a).
In these cold-cathode electron sources, a high electric field is applied between the gate and the emitter so that the emitter can emit electrons, and a positive voltage is applied to the anode so that it can collect the emitted electrons. This resulted in the problem of spreading of the emitted electrons, due to the fact that the anode-gate electric field is weaker than the gate-emitter electric field.
In recent years, electron sources for depletion-mode electron emission apparatus have been proposed, as disclosed in Japanese Patent Laying-open Publication (Unexamined Application) No. 5-282990, for example. In these types of electron sources, a material, such as diamond, that emits electrons in low electric field is used in the emitter, and electrons are drawn from the emitter by applying a voltage to the anode, while using the gate electrode for controlling the emission of electrons.
Japanese Patent Laying-open Publication (Unexamined Application) No. 2000-156147 discloses a cold-cathode electric field emission device including an anode, gate and emitter. With this device, electrons are emitted by an electric field between the anode and the emitter, and the electron beam is focused by an electric field between the gate and the emitter. The area of the gate opening is larger than that of the bottom portion of the gate. The publication also describes the conditions of isoelectric lines irrespective of the structure.
Various materials for the electric-field emission electron source that is used in FEDs are known. Conventional materials require an electric field intensity of 1000 V/μm as an effective value to obtain sufficient electron emission. Thus, a value on the order of 100 V/μm is obtained for the intensity of an actually applied electric field by the above-mentioned structure to increase the electric field concentration constant.
Carbon materials such as carbon nanotubes are also gaining attention as electron emission materials, for they have been confirmed to emit electrons with an extremely small electric field intensity. Uemura et al. of Ise Electronics Corporation have proposed (in SID 98 DIGEST, pp. 1052–1055) an electric field emission electron source in which carbon nanotube is used in the emitter and with a gate electrode formed in the shape of a mesh or a grid, as shown in FIG. 8(e).
In the conventional cold-cathode electron source having a protruding electron emission portion, the spreading of electrons is prevented by providing a focus electrode such as shown in FIG. 8(b), as disclosed by Ito et al. of Futaba Corporation in Japanese Patent Laying-open Publication (Unexamined Application) No. 7-29484. This arrangement resulted in an increased number of manufacturing steps and complication of the device structure.
When a material that can easily emit electrons is used in the emitter, a sufficient amount of electrons can be emitted with an anode-emitter electric field. Thus, a depletion mode may be employed for operation, as in Japanese Patent Laying-open Publication (Unexamined Application) No. 5-282990. The depletion mode is a technique for controlling the emission of electrons from the emitter by applying an emission-suppressing voltage to the gate electrode, thus narrowing the passageway of electrons. Accordingly, there is no electron emission in an emitter region near the gate, and the strong electric field region is limited to the emitter near the gate hole center, as shown in FIG. 8(c), thus narrowing the region of the emitter where emission takes place and lowering the emitter utilization efficiency.
In Japanese Patent Laying-open Publication (Unexamined Application) No. 2000-156147, the gate electrode is used for focusing an electron beam. As the area of the gate opening is larger than the area of the bottom surface of the gate, as shown in FIG. 8(d), it is difficult to completely suppress the electric field from the anode. The production process is also complicated. The conditions regarding the isoelectric lines concern only general conditions about focusing, and an accurate analysis is not described.
In the example of SID 98 DIGEST, pp. 1052–1055, because it employs a mesh- or grid-like gate electrode as shown in FIG. 8(e), it is difficult to bring the gate electrode closer to the emitter. And because the emitter is present at locations other than the position immediately below the gate opening, the current flowing through the gate electrode increases. This results in reduced efficiency, for the electrons other than those reaching the anode electrode and allowing a phosphor to irradiate are wasted.
It is therefore an object of the invention to provide an inexpensive cold-cathode electron source capable of improving the utilization efficiency of an electron beam, which can be realized by a simple structure, and a field emission display utilizing the electron source.